Primary biliary cirrhosis (PBC) is a chronic disease characterised by progressive inflammatory obliteration of the intrahepatic bile ducts. The disease is marked by an autoantibody response to mitochondria.sup.1-4, originally identified using immunofluorescence.sup.5. With the recent use of immunoblotting, specific proteins have been recognized as targets of the anti-mitochondrial antibodies (AMA) of PBC.sup.2,6,7. In particular, serum antibodies to a 70 kilodalton (kd) protein have been found in greater than 95% of patients with PBC but not in patients with other autoimmune liver diseases,.sup.2,8 ; two other proteins of 45 and 39 kd are less frequently detected in PBC sera.sup.1,2,9. The identity of each of these autoantigens has been unknown, as is the relationship of these antigens to the pathogenesis of the disease. However, the 70 kd antigen is highly conserved in evolution, being present in mammals, yeast and bacteria.sup.10 and it is therefore believed to have an important structural or biological function.sup.2.
Despite the paucity of data on mechanisms of anti-mitochondrial antibody formation, by enzyme-linked immunosorbent assay (ELISA), clinically more than 95% of patients with PBC can be found to have such anti-mitochondrial antibodies.sup.2,6. When crude mitochondrial antigen preparations are used, subjects with a variety of diseases, including patients with liver diseases other than PBC, certain connective tissue diseases, and drug reactions, and occasionally even healthy individuals, can also be demonstrated to have antibodies to mitochondria. Accordingly, assays using such crude preparations are unable to provide specific diagnosis of PBC. By way of example, German Patent Publication No. 3,237,602 discloses an ELISA for detection and determination of antimitochondrial antibodies in serum using a crude mitochondrial antigen preparation. The lack of specificity of the assay is evident from the suggested use of the assay in the specific diagnosis of disorders such as PBC as well as the cholestatic form of chronic active hepatitis, syphilis (II), drug-induced pseudo lupus erythematodes syndrome, certain primary non-hepatic immunopathies, iproniazid-induced hepatis and side effects of certain medicaments such as beta-receptor blockers. By more vigorous isolation of mitochondrial membranes, the problem of antigenic heterogeneity becomes clearer and has led to definitions of specific mitochondrial antigens based on trypsin sensitivity and location of antigens within inner vs outer mitochondrial membranes. Notwithstanding this, however, the diagnosis of PBC still relies heavily on the demonstration of anti-mitochondrial antibodies by the relatively insensitive procedure of immunofluorescence or by more sensitive, but still relatively nonspecific, methods, including complement fixation, ELISA, and immunoprecipitation.sup.23-28.